And it is a trick. This isn't something that you can enable in a menu; you have to be using the default web browser and catch it at just the right time.
You can enable the partial refresh mode by opening the web browser and using pinch-zoom. Right after you do that, and before it has a chance to do full screen refresh, select the pull down menu from the top of the screen. If you're quick enough then you should be able to keep the mode when you exit and open another app.
BTW, don't go back to Sony's home screen or the stock reading app. This will kick you out of the partial refresh mode.
I've set aside my Sony Reader Wifi for the past few weeks while I reviewed other devices, and I cannot wait to get back to it. Underneath Sony's reading app lies Android v2.2 Froyo, and it only takes a simple hack to access it. Once you do, you'll have an Android tablet with a 6" E-ink screen, Wifi, microSD card slot,and limited support for third party apps (not enough room). I've taken to showing the Sony Reader Wifi off as my new Kindle.
And now that I have this partial refresh mode, it's going to get a lot better.